This is less of a full soft robot than it is just a "component" to one. If you're familiar with the Soft Robotics Toolkit (softroboticstoolkit.com), then chances are you're familiar with the PneuNets Bending Actuator, which is a soft robotic "component" that, as it's name implies, bends. I've taken the basic principles behind this and modified it so that it exhibits linear motion as opposed to curving motion as seen in the PBA. I call my variation of this design the PneuNets Extending Actuator (PEA). I have fabricated this through use of 3D printed molds and Ecoflex 30 (Same material used in the Glaucus). Here are some pictures: http://imgur.com/ocsmkuOhttp://imgur.com/6LZS3XL
As this is still an ongoing project of mine, I am looking for ways to improve it. For example: unlike the PBA, the PEA does not have a strain-limiting layer, however, my actuator could benefit from one, but only if the layer was extensible but not bendable. Not really even sure if something like that exists . As I continue with this project I will probably post more updates.

Excellent work. Thanks for posting. Any ideas on what you'll use them for?

If you're specifically looking to perfect an extending actuator to act like a piston in a soft robotics context, you might also play with other geometries and materials. You'll notice that the PneuNets bots all have chambers that are perpendicular to the direction of bend. I've had some success with extending actuators that have chambers all running parallel lengthwise. It might also be useful to source single direction stretch fabrics. If you build a cylinder with no interior chambers but contain it in a pocket of material that only stretches lengthwise you'll get the same effect. It'd be like a McKibben muscle in reverse.